1. Field of the Invention
The present invention relates to a metal shaft, a golf shaft using the metal shaft, a golf club using the golf shaft, a method of manufacturing the metal shaft and a tempering apparatus.
2. Description of the Related Art
A conventional metal golf shaft as an example of a metal shaft is disclosed in, for example, JP7005-34517A.
The metal golf shaft is made by carrying out quenching treatment to a tubular steel material having high strength and toughness according to an alloy design so as to transform the same into a martensite structure and then tempering the tubular steel material so as to recover the toughness. This achieves the strength and durability of the golf shaft.
The tempering after the quenching applies heat to the tubular steel material at a uniform temperature in a longitudinal direction at once, thereby to set an approximately constant elastic coefficient over the entire length of the finished golf shaft.
Meanwhile, ability, swing style and the like vary depending on each user and therefore a golf shaft may be required to be suited for each user. For this, it is required to prepare a plurality of variations on the golf shaft, the variations having different wall thicknesses and outer diameters etc. This causes increase in the number of variations on a shape of the golf shaft.
In this case, different processing apparatuses and different processing jigs such as dices and plugs are required to manufacture different golf shafts according to the number of variations on the shape and adjustment better suited for the respective apparatuses and jigs are also required.
Obviously, the number of man-hours for designing the different golf shafts increases according to the number of variations on the shape, to cause a problem of increase in manufacturing cost and time.
Further, it becomes hard to stabilize quality of the different golf shafts as the number of variations on the shape increases. It is a task to provide a product suited for each user while maintaining high quality.
The tempering after the quenching applies heat to the tubular steel material at a uniform temperature in the longitudinal direction as mentioned above, thereby to set an approximately constant tensile strength over the entire length of the finished golf shaft. The golf shaft, therefore, may break or bend by overload that exceeds design stress.
In particular, a joint between a head and the shaft is likely to cause stress concentration and be broken. To prevent such a defect, it is required to thicken a wall thickness or increase an outer diameter at the joint.
This configuration, however, weights a part of the golf shaft to lose a balance of the golf shaft and affect a swing.
Such a problem may be caused a metal shaft product other than the golf shaft such as bat for baseball or softball, ski pole, trekking, pole, and TV antenna for which hardness may be set by tempering after quenching. For example, in a case where such a shaft product has an approximately constant elastic coefficient or tensile strength set over the entire length, it involves increase in the number of variations on a shape when designing different variations according to external force.